Electromagnetic flowmeter

ABSTRACT

A noninvasive electromagnetic flowmeter particularly adapted to measure arterial blood flow in human beings is provided. A homogeneous magnetic field is produced in the region of the artery under measurement by means of at least one electromagnetic coil or a permanent magnet suitably positioned near the human being. Blood flow induced signals are sensed by electrodes placed on the skin adjacent the artery and fed to a measurement and control circuit. Electrocardiogram signals are sensed by a second pair of electrodes placed on the body near the heart and synchronize the operation of this system. One embodiment of the control and measurement system comprises a pair of auxiliary electrodes located where a strong and sharp cardiogram pulse can be repeatedly obtained to be used as a synchronizing signal and as a clock, at least one coil of large enough size having enough turns of wire to be adequate for the production of a strong and homogeneous magnetic field in the region of the artery under study, a source of D.C. current to feed that coil, means to reverse that D.C. current after a given number of heart cycles in accordance with a program controlled by the sychronizing signal, a pair of measuring electrodes placed on the skin adjacent the artery, an amplifier to amplify the signal appearing between said measuring elecrodes, with filtering means to eliminate the D.C. unbalance of the electrodes and the high frequency noise, means to paralyze the amplifier and to reverse its output during current reversal, and means synchronized by the synchronizing signal to average the pulsatile signal measured between the measuring electrodes during successive heart cycles so as to extract the repetitive wave shape of the blood flow pulses from the large random noise in which it is otherwise buried. In another embodiment of the control and measurement system, the coil in which a stable current is maintained during all the measuring intervals, as described in the paragraph above, is replaced by an electrically reversible permanent magnet. This permanent magnet is surrounded by a magnetizing coil in which DC current pulses of short duration are passed only at the times when the polarity of the magnetic field, and therefore the polarity of the permanent magnet, have to be reversed. There is no current in the magnetizing coil during the intervals when measurements of blood flow cycles are made and averaged. In still another embodiment of the control and measurement system, a permanent magnet is also used, but the reversal of the magnetic field, at the times when it is needed, is obtained by mechanically reversing or displacing the permanent magnet.

United States Patent 11 1 Doll et al. I

[ 1 Sept. 18, 1973 ELECTROMAGNETIC FLOWMETER [75] Inventors: HenriGeorges Doll, New Yorlqllans J. Broner, Glendale, both of NY.

[73] Assignee: Doll Research, Inc., New York, N.Y.

[22] Filed: July 1, 1971 [21] Appl. No.: 158,697

Related US. Application Data [63] Continuation-in-part of Ser. No.66,240, Aug. 24,

l970, Pat. No. 3,659,59l.

52 us. 01. 128/2.05 F, 73/194 EM 511 1111. c1. A6lb 5/02 58 Field ofSearch 128/205 F, 2.05 R, 128/205 v, 2.05 P, 2 R; 73/194 EM [56]References Cited UNITED STATES PATENTS 3,659,591 5/1972 Doll et al128/205 F 2,924,213 2/1960 Fleck 128/2.1 E 3,060,923 10/1962 Reiner128/21 E 2,867,119 1/1959 Sturgeon @1111 73/194 EM 3,131,560 5/1964Cushman et a1. 73/194 EM 3,184,966 5/1965 Thornton et al..... 73/194 EM3,377,855 4/1968 Coia et al. 73/194 EM OTHER PUBLICATIONS Abel; F.L.,IRE Trans. on Med. Electronics, Dec., 1959, pp. 216-219. Spencer; M.P.et al., IRE Trans. on Med. Electronics, Dec., 1959, pp. 220-227.

Primary Examiner-Kyle L. Howell Attorney-Kenyon & Kenyon Reilly Carr &Chapin [5 7] ABSTRACT A noninvasive electromagnetic flowmeterparticularly adapted to measure arterial blood flow in human beings isprovided. A homogeneous magnetic field is produced in the region of theartery under measurement by means of at least one electromagnetic coilor a permanent magnet suitably positioned near the human being. Bloodflow induced signals are sensed by electrodes placed on the skinadjacent the artery and fed to a measurement and control circuit.Electrocardiogram signals are sensed by a second pair of electrodesplaced on the body near the heart and synchronize the operation of thissystem.

One embodiment of the control and measurement system comprises a pair ofauxiliary electrodes located where a strong and sharp cardiogram pulsecan be repeatedly obtained to be used as a synchronizing signal and as aclock, at least one coil of large enough size having enough turns ofwire to be adequate for the production of a strong and homogeneousmagnetic field in the region of the artery under study, a source of D.C.current to feed that coil, means to reverse that DC. current after agiven number of heart cycles in accordance with a program controlled bythe sychronizing signal, a pair of measuring electrodes placed on theskin adjacent the artery, an amplifier to amplify the signal appearingbetween said measuring elecrodes, with filtering means to eliminate theDC. unbalance of the electrodes and the high frequency noise, means toparalyze the amplifier and to reverse its output during currentreversal, and means synchronized by the synchronizing signal to averagethe pulsatile signal measured between the measuring electrodes duringsuccessive heart cycles so as to extract the repetitive wave shape ofthe blood flow pulses from the large random noise in which it isotherwise buried.

In another embodiment of the control and measurement system, the coil inwhich a stable current is maintained during all the measuring intervals,as described in the paragraph above, is replaced by an electricallyreversible permanent magnet. This permanent magnet is surrounded by amagnetizing coil in which DC current pulses of short duration are passedonly at the times when the polarity of the magnetic field, and thereforethe polarity of the pennanent magnet, have to be reversed. There is nocurrent in the magnetizing coil during the intervals when measurementsof blood flow cycles are made and averaged.

In still another embodiment of the control and measurement system, apennanent magnet is also used, but the reversal of the magnetic field,at the times when it is needed, is obtained by mechanically reversing ordisplacing the permanent magnet.

7 Claims, 34 Drawing Figures H MEASUREMGNI' an 6'0 TAOL SYSTEMPAIENIEnssmm 3.759.247

SHEET 1 BF 6 masunmavr I cow mac .s-rsrsM 14 FIG! FIGIa sysiz-w FROM L06/6 I w CL [PP/N6 RA MP I CIRCUIT OUTPUT PAIENIEU EM 3759.247

sum 6 or 6 ELECTROMAGNETIC FLOWMETER CROSS REFERENCE TO RELATED PATENTAPPLICATION This is a Continuation-in-Part application of United StatesPatent Application Ser. No. 66,240, filed on Aug. 24, 1970, by HenriGeorges Doll and Hans J. Broner and entitled, ELECTROMAGNETIC FLOWMETERnow U. S. Pat. No. 3,659,591.

FIELD OF THE INVENTION This invention relates to electromagneticflowmeters and more particularly to a transcutaneous electromagneticflowmeter for monitoring the flow pulse in the blood vessels of livingbeings.

BACKGROUND OF THE INVENTION In general, electromagnetic flowmeters forthe measurement of the blood flow have been invasive, i.e., they haverequired surgical exposure of the blood vessel under measurement andimplantation of at least part of the sensing device about such vessel.Such meters are severely limited in their application and not suitablefor clinical use due to the necessity of such surgical procedures andthe attendant sterility problems. In addition, not all blood vessels maybe exposed in this way since those that have become arteriosclerotic arebrittle and may be damaged in the implanting procedure. The ugly scarscaused by the surgical procedure also dictate against the use ofinvasive meters except in the most critical cases. For these reasons,the invasive flowmeters are rarely used on human beings and are mostlyused for experiments on anesthetized animals.

Besides these physical limitations in known electromagnetic flowmeters,such meters have also had severe drawbacks in the electronic sensing andmeasurement system. Since the desired blood flow signal is mixed in withunwanted noise signals created in the body and in the electronic systemitself, it has been found difficult to eliminate the noise in order toobtain an accurate blood flow reading. Such noise includes extraneoussignals caused by poor placement of the flowmeter around the vesselunder measurement, noise created by electrical interaction of theelements of the flowmeter sensor and between the sensor and the tissuewith which it is in contact, quadrature effect in sine wave typeflowmeters and transformer spikes in square wave type flowmeters.

It is desirable that the blood flow measurement by the flowmeter betaken during a period of constant magnetic field, and that the flowmeterbe simple to operate and be capable of monitoring the blood flow in anumber of vessels in the same individual.

OBJECTS OF THE INVENTION It is thus an object of the present inventionto provide a non-invasive electromagnetic flowmeter which measures theblood flow in the vessels in living beings without surgical implantationof sensors in the being.

It is a further object of the present invention to provide anon-invasive electromagnetic flowmeter which provides an averagedmeasurement of the blood flow pulse over a number of heart cycles.

It is another object of the present invention to provide a non-invasiveelectromagnetic flowmeter which effectively cancels out noise created bythe electrocardiogram signal between the blood flow measuring electrodesover a number of heart cycles and which eliminates random noise duringthat number of cycles by averaging the pulses.

It is yet another object of the present invention to provide anoninvasive electromagnetic flowmeter wherein the measurement andcontrol system is synchronized by means of a reference electrocardiogramsignal.

It is still another object of the present invention to provide anelectromagnetic flowmeter wherein flow induced signals are only measuredwhile the magnetic field is constant and wherein the measurement systemis paralyzed during reversal of the magnetic field.

It is still a further object of the present invention to provide anon-invasive electromagnetic flowmeter which is simple to operate andwhich may be used to monitor the blood flow in a plurality of vesselssimultaneously or sequentially.

SUMMARY OF THE INVENTION According to a first aspect of the presentinvention, a homogeneous magnetic field is created in the region of theblood vessel under measurement by magnetic field producing meanssituated external of the living be ing. Blood flow induced electricalsignals are sensed by means of sensors placed on the skin of the beingin the vicinity of the vessel.

According to one other aspect of the invention, these signals are fed toa measurement and control system whose operation is synchronized bymeans of electrocardiogram signals sensed by a second set of sensorsplaced on the skin of the being in a location where theelectrocardiogram signal is strong and sharp, as for example, in thevicinity of the heart.

In one embodiment of the measurement and control system, anelectromagnetic coil provides a substantially homogeneous magnetic fieldat the artery to be studied, the coil being fed a stable DC currentduring blood flow measurement. A plurality of flow induced signals aremeasured by the first set of sensors and are averaged in an analogmeasuring circuit to produce an averaged pulse which is a function ofthe rate of flow in the artery under measurement. The electrocardiogramsignals detected by the second set of sensors are utilized to reversethe field of the electromagnet through a suitable control circuit, toreverse the polarity of the analog measuring circuit, and to paralyzethe measuring circuit during these reversals.

These reversals are accomplished after a given number ofelectrocardiogram pulses in order that an equal number of cardiogramsignals of opposite polarity be stored in the analog circuit thuscausing cancellation of the electrocardiogram component of the flowinduced signal in the analog measuring conduit. Due to the random natureof the other noises, they are eliminated by being averaged out in theanalog measurement circuit.

Another embodiment of the measurement and control system includes, asthe means for providing the stable and sufficiently homogeneous magnetfield at the artery to be studied, an electrically reversible permanentmagnet consisting of a permanent magnet core surrounded by a magnetizingcoil. After a predetermined number of signals have been measured andaveraged, a DC current pulse of short duration and of the properpolarity and intensity is applied to the magnetizing coil to demagnetizethe core and to immediately remagnetize it with the opposite polarity,after which the current is interrupted and a new series of thepredetermined number of signals is measured and added in an averagingcircuit. In this embodiment, no current is applied to the magnetizingcoil, except at times of reversals. During the measuring intervals, themagnetic field is due only to the permanent magnetization of the magnet.

Still another embodiment of the measurement and control system includes,as the means providing the substantially homogeneous magnetic field atthe artery to be studied, a permanent magnet which is mechanicallydisplaced between two positions so as to reverse the polarity of suchmagnetic field.

According to a further aspect of the invention, the analog measuringcircuit is paralyzed during magnetic field reversal.

It is to be understood that, as used herein, the term noninvasiveelectromagnetic flowmeter is intended to mean a flowmeter whereinsurgical exposure of the blood vessel under measurement is not required,and there is no surgical implantation of parts of the sensing deviceabout the blood vessel. However, the term noninvasive electromagneticflowmeter does include a flowmeter wherein the sensing electrodes areeither placed on the surface of the skin or are implanted in the tissuewithout requiring surgery, such as by injection with a hypodermicsyringe.

DESCRllPTION OF THE DRAWINGS FIGS. 1 and in are diagrammatic views of apreferred embodiment of electromagnetic flowmeter according to thepresent invention illustrating measurement of blood flow respectively inthe carotid and femoral arteries of a human being.

FIG. 2 is a block diagram of a preferred embodiment of measurement andcontrol electrical system of the flowmeter of FIG. 1.

FIGS. 3a and 3b are illustrative block diagrams of the control logiccircuitry of FIG. 2

FIG. 4 is an illustrative schematic diagram of the trapezoid functiongenerator of FIG. 2.

FIG. 5 is an illustrative schematic diagram of the current source ofFIG. 2.

FIG. 6 is a waveform diagram of one of the signals generated in thecircuit of FIG. 4.

FIGS. 7a-g are waveform diagrams of several of the signals generated inthe control logic circuit of FIG. 30.

FIG. 8 is a representative waveform diagram of the PQRSTelectrocardiogram signal of the human heart.

FIGS. 9a-e are waveform diagrams of various signals generated in thecircuitry of FIG. 2.

FIG. 10(a) is a waveform diagram of an actual electrocardiogram signalof a human heart measured between two electrodes across the femoralartery.

FIG. 10(b) is an example of the residual noise signal after reversingand averaging a wave of actual electrocardiogram signals and othernoises over a period of approximately 10 minutes in the flowmeter ofFIG. 1 but without applying the magnetic field, and therefore withoutblood flow signals.

FIGS. 10(c)-(h) are waveform diagrams of averaged blood flow pulses asmeasured in the femoral artery of a human being.

FIG. ll is a sectional elevational view of a skin electrode that may beused according to the present invention.

FIG. 12 is a diagrammatic view illustrating the electromagneticcharacteristics of blood flow in an artery.

FlG. 13 is a schematic diagram of an electrical circuit that may be usedto correct for movement of a patient during blood flow measurementaccording to the present invention.

FIG. 14 is a perspective view of an electrically reversible permanentmagnet employed to provide the homogeneous magnet field in the region ofthe artery under study, illustrative of another embodiment of theinvention.

FIG. 15 is a diagrammatic view of a permanent magnet which is rotatablymounted for producing a mechanically reversible magnetic field in theregion of the artery under study, illustrative of still anotherembodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS Referring now to the drawings,there is shown in FIG. 1, a general diagrammatic view of a preferredembodiment of blood flowmeter according to the present invention used tomeasure the blood flow rate through the carotid artery of a patient. Asshown, a patient 10 is lying in a prone position on an examination table12. Since it is desired to measure the blood flow rate through the rightcarotid artery 14 of the patient, a ho mogeneous magnetic field iscreated in the region of artery 14 by means of electromagnet 18. Thestrength of this magnetic field must be sufficiently large so that adetectable electrical signal induced by the passage of blood in thecarotid artery through the magnetic field is present at the skin surfaceof the neck.

Electromagnet 18 is thus comprised of a coil of heavy gauge wire adaptedto carry large amounts of current to create a high intensity magneticfield. It has been found that a substantially homogeneous magnetic fieldis produced in the region of the artery under measurement by employingan electromagnet coil having a di ameter which is at least twice thedistance of the coil from the artery. A homogeneous magnetic field is desirable in order to avoid false readings that may occur if the patientmoves his head and thereby changes the location or orientation of theartery in the field. It is also desirable that the magnetic field beroughly perpendicular to the skin and the artery.

In order that measurement may be made of blood flow in differentarteries of the patient it may be desirable that more than oneelectromagnet such as 18 be provided, as, for example, one for thecarotid arteries region and another one for the femoral arteries region.

Current is supplied to electromagnet 18 through electrical conductors 20from measurement control system 24. A pair of electrodes 26 is placed onthe neck of the patient 10 on either side of the right carotid artery.Electrodes 26 sense an electrical signal that has been induced by theflow of blood in the carotid artery through the homogeneous magneticfield present in the region of the artery. This induced signal is fed tosystem 24 by means of shielded twisted wire pair 28 connected betweenelectrodes 26 and terminals 30 of system 24.

A second pair of electrodes 26a is placed on the chest of patient 10 andsense the large electrocardiogram signals produced by the heart at thatlocation. Shielded twisted wire pair 32 connected between chestelectrodes 26a and terminals 34 of system 24 carries these signals tosystem 24.

Since it is desirable that system 24 be at the same ground potential aspatient 10, ground electrode 26b is placed on the leg or arm'of patientand connected to ground terminal 36 of system 24 by conductor 38.Electrode 26b is preferably placed on a portion of the body which isremoved from electrodes 26 and 26a.

The blood flow diagram produced by system 24 may be recorded on a stripchart recorder 40, may be displayed on a visual display device such asan oscilloscope (not shown) or may be recorded on a suitable recordingmedia such as magnetic tape eigher directly or after being converted todigital data for use in a digital computer by well known analog-digitalconversion devices.

FIG. la shows the relative positioning of electromagnet 18 for themeasurement of blood flow through the right femoral artery 42 of patient10 and also the relative positioning of electrodes 26 for sensing theflow induced signal.

In general, the blood flow induced signal sensed by electrodes 26 willcontain a number of noise components in addition to the component whichis a function of the blood flow in the artery. The main noise componentis the electrocardiogram signal produced by the heart which can have astrength many times that of the flow induced signal especially at anartery close to the heart such as the carotid artery.

It has been found that, since it is not necessary to display eachindividual blood flow pulse, these pulses with proper synchronizingbased on the reference cardiogram may be averaged over a long period,such as one or several minutes to progressively build up an averagepulse with the wave form cleared from noise. The cardiogram noisecancels out because all the cardiogram pulses are identical andsynchronized in the averager, but one half of them with one polarity,and the other half of them with the opposite polarity. The random noisedoes not build up in the averager because it is not synchronized, andits effect becomes practically negligible if the averaging lasts longenough.

Referring now to FIG. 2 there is shown a preferred embodiment ofmeasurement and control circuit 24. The blood fiow induced signal sensedby electrodes 26 is fed by wire pair 28 to the input terminals 30 ofpreamplifier 44. As described above, a common ground is maintainedbetween the patient and circuit 24 by means of electrode 28b placed onthe skin of the pa tient, ground wire 38 and ground terminal 36 ofpreamplifier 44. After the flow induced signal has been amplified bypreamplifier 44 it is further amplified by D.C. amplifier 46. Thissignal is then filtered by low pass filter 48 which filters out all highfrequency noise components including any noise component that may beproduced by the power line frequency.

The electrocardiogram signal sensed by electrodes 26a is fed toterminals 34 of preamplifier 50 by twisted wire pair 32. After thecardiogram signal has been amplified by preamplifier 50 it is fed toaverager 52 and thence to control'logic circuit 54 to be used tosynchronize the operation of the various functions of system 24.

A reversible steady state D.C. current is supplied to field coil 56 ofelectromagnet 18 by means of trapezoid function or ramp generator 58connected to current source 60 which in turn supplies current to coil56. Generator 58 is connected to control logic circuit 54 which controlsthe operation thereof.

After the blood flow signal is filtered by filter 48, it is fed througha dead time relay 62 and a double pole double throw (DPDT) relay 64 todifferential amplifier 66 where it is amplified and then fed to averagermeasuring circuit 52. Relays 62 and 64 are also connected to andcontrolled by control logic 54.

In a known type of analog measuring averager, the Model T DH-9 WaveformEductor manufactured by Princeton Applied Research Corporation ofPrinceton, New Jersey, a repetitive input waveform to be measured isdivided into increments of equal duration and stored in a 100 channelcapacitor memory. After a sufficient number of repetitions the voltagelevel on each capacitor will be proportional to the average value forthat segment of the waveform. Random noise and non-synchronous signalsare eliminated and the desired signal can be read out on a chartrecorder or oscilloscope.

The operation of the electromagnetic flow meter as shown in FIGS. 1 and2 may be understood by referring to the waveform diagrams of FIG. 9.FIG. 9a shows a series of reference electrocardiogram signals having thegeneral shape of the waveform PQRST shown in FIG. 8. Theelectrocardiogram signal is characterized by a sharp spike at R which isused to trigger measurement and control system 24 and synchronize theoperation of the various components thereof. This synchronization on theR line is desirable since the heart beats at irregular intervals. Ingeneral, the sequence of operation of the flowmeter is caused to berepeated after a given number of heart cycles. In the example shown inFIG. 9, this repetition occurs every 16 heart beats.

At RNo.0 the control logic circuit 54 causes the current in field coil56 to be reversed by reversing trapezoid function generator 58 and alsocauses the polarity of the measuring circuit to be reversed byactivating relay 64. Control logic circuit 54 also causes relay 62 tostay open from RNo.0 to RNo.2 so that no signals are fed to averager 52during this period of time. This is to prevent unwanted noise caused bythe field reversal to be fed to averager 52 and to allow the system toreturn to a steady state condition before measurement of the blood flowsignals is resumed. As shown in FIG. 9d the current is reversed at RNo.0and the system allowed to stabilize between RNo.1 and RNo.2.

At RNo.2, control logic circuit 54 causes relay 62 to close (FIG. 9b)thus allowing the blood flow signals to be fed into averager 52. FIGS.9(c) and 9(e) show the two main components of the signal, 9(e) showingthe electrocardiogram noise component which is to be cancelled out andFIG. 9(e) the desired blood flow induced component which is to bemeasured.

During the interval from RNo.2 to RNo.8, a steady state D.C. current issupplied to coil 56 of electromagnet 18 which in turn creates a stableand uniform homogeneous magnetic field in the region of the artery (orarteries) under measurement. Since the averager 52 is operated insynchronism with the flow induced signals by means of the referenceelectrocardiogram signals sensed by electrodes 26a, six flow inducedpulses will be introduced into averager 52 during this interval.

As shown, the direction of the magnetic field and polarity of relay 64cause 6 pulses having 6 negative cardiogram components (FIG. 9(a)) and 6positive blood flow components to be loaded on the condensers ofaverager 52.

At RNo.8, control logic circuit 54 causes generator 58 to reverse, thuscausing the current through coil 56 to reverse and the magnetic field toreverse. Reversal of the polarity of the magnetic field causes areversal of the polarity of the magnetic field-induced blood flowcomponents, but does not affect the polarity of the electrocardiogramcomponents. Circuit 54 also causes relay 64 to operate in order toreverse the polarity of the signals fed to averager 52 and causes relay62 to be opened between RNo.8 and RNo.10 to prevent the feeding of bloodflow signals to averager 52. The relay operating times are chosen sothat the deadtime relay operates before the others.

By RNo.9 (FIG. 9(a')), the field current has reversed and it iscompletely stabilized at RNo.10.

At RNoJlll, control logic circuit 54 causes relay 62 to close, to permitblood flow pulses to be fed to averager 52 once again. During theinterval from RNo.l to RNo.l6, six signals are fed to averager 54.However, since the magnetic field has been reversed as well as thepolarity of the electrical signals fed to averager 52, the six signalswill have six positive electrocardiogram components and six positiveblood flow components. This is because the positive blood flowcomponents were reversed in polarity of a total of two times; first, dueto a reversal in the polarity of the magnetic field, and second, due tothe electrical switching of the polarity of the electrical signals fedto the averager 52. The positive electrocardiogram components werereversed in polarity a total of only one time, this reversal beingproduced by the electrical switching of the polarity of signals fed toaverager 52. These six positive cardiogram signals will cancel out thesix negative cardiogram signals fed in during the interval from RNo.2 toRNo.8 thus eliminating the cardiogram noise component from the desiredflow pulse. The six positive blood flow components will be added to thepreviously obtained six positive blood flow components to produce twelveblood flow pulses loaded on the condensers of averager 52 free ofelectrocardiogram noise.

At RNo.I6, control logic circuit 54 causes a repetition of events whichtook place at RNo.0.

Thus, during a cycle of 16 heart beats, 12 pulses have accumulated inaverager 52 and the signals during four of the 16 heartbeats have beenignored. Measurement has only taken place when the magnetic field isconstant and has been ignored during periods of current reversal whenconditions are unstable. In this manner, an accurate measurement of theblood flow pulse is ob tained and reliability of the measuring circuitis assured.

This cycle is repeated over a long enough period of time to give a goodaverage reading of the blood flow pulse in the artery under measurement.Since the average heart beats in the range of from 60 to 100 beats perminute (under 2 ha.) a reading over a period of 1-10 minutes shouldprovide an averaged blood flow waveform which is a function of theactual blood flow in the artery under measurement.

As an example, FIG. in which the signal scale is 0.1 microvolt perdivision for all curves, shows the results of a number of actualmeasurements of blood flow in the right femoral artery of a man. FIG.10(a) is the waveform of the averaged electrocardiograms recordedbetween the electrodes across the femoral artery in the absence ofmagnetic field. FIG. 10(b) is the residual cardiogram noise signal afterreversing and averaging equal numbers of positive and negativecardiogram pulses and other noise over a period of approximately tenminutes also in the absence of magnetic field.

FIGS. l0(c)(e) are resultant blood flow waveforms of three differentmeasurement periods of approximately 10 minute durations. FIGS. 10(j)and (g) are resultant waveforms of 560 averaged pulses and FIG. 10(h) isthe resultant waveform of 280 averaged pulses.

It will be noted that the six waveforms are substantially similar andthus may be said to provide a good indication of the blood flow pulse inthe artery being measured.

When measurements are taken at periodic intervals, a decrease in thepeak to peak amplitude between the positive and negative peaks wouldindicate that the blood flow in the artery was decreasing, thus givingan important warning to the physician.

Referring again to FIG. 10(c) through (e), the blood flow pulses havebeen added in the averager for a fixed period of time, 10 minutes inthat example, and under these conditions the deflections of the curvestend to be proportional to the total flow during the correspondingaveraging period of 10 minutes. This means that any change of amplitudeof the curve deflections from one recording to another recorded later onwill tend to represent the change of the volume of flow per minute fromthe time of the first recording to the time of the second recording.

Contrarywise, in the FIGS. 10(f) through (h), the blood flow pulses havebeen added in the averager for a fixed number of pulses, 560 pulses forFIG. 10(1) and FIG. 10(g), and under these conditions the deflections ofthe curve tend to be proportional to the total flow for thecorresponding fixed number of pulses. This means that any change ofamplitude of the curve from one recording to another curve recordedsubsequently will tend to represent the change of volume of flow perpulse, generally referred to as the stroke volume, from the time of thefirst recording to the time of the second recording.

Since it is generally more practical for the simplicity of the controlsystem to average over a fixed number of pulses rather than over a fixedtime, it is the first alternative that will normally be used. It ishowever possible, even in that case, to obtain a curve identical withthe one that would be obtained by adding pulses over a fixed period oftime, the deflections of which would then be proportional to the volumeof flow per minute. This can be obtained by making the recording scaleproportional to the average heart rate, i.e., number of heart beats perminute, during the averaging interval by providing adequate gain controlin the recording circuit. In some cases it might even be advantageous,to facilitate quick diagnosis, to record the blood flow pulses on bothof the scales described above, in which case one recording will tend torepresent the flow per minute while the other recording will tend torepresent the stroke volume.

Where blocking condensers are used in the preamplifiers and amplifierswhich amplify the blood flow signal, control logic circuit 54 will causepreamplifier 44 to be paralyzed during current reversal to preventcharging of the condensers by the transients produced during reversaland the residual discharge thereof during the measuring intervals.

Referring now to FIG. 3a, there is shown in greater detail one circuitwhich may be used for control logic circuit 54 to control the operationof system 24 over the 16 heart beat cycle described hereinabove and toFIG. 7 which shows waveforms of various signals produced in the circuitof FIG. 3a. As shown in FIG. 30, four flip-flops 66a, 66b, 66c and 66dare connected in sequence and produce the voltage signals shown in FIG.7.

FIG. 7a shows the voltage X, fed to the input of flipflop 66a, thisvoltage comprising a series of square pulses which are of the samefrequency as and which have been formed from the R spikes of theelectrocardiogram signals sensed by electrode 26a.

As is well known, the voltage signal produced by each successiveflip-flop in a chain will have a frequency one half the frequency of thetrigger voltage signal applied to the input of the flip-flop. Thus, thevoltage X (FIG. 7b) produced by flip-flop 660 has one half the frequencyof triggering voltage X the voltage X (FIG. 7c) produced by flip-flop66b has one half the frequency of triggering voltage X,; the voltage X(FIG. 7d) produced by flip-flop 660 has one half the frequency producedby triggering voltage X and the voltage X (FIG. 7e) produced byflip-flop 66d is one half the frequency of triggering voltage X Voltagesignal X is supplied to relay amplifier/driver 68 which triggers DPDTrelay 64 and to driver 70 which triggers generator 58.

Voltage signal X (FIG. 7b) is derived from flip-flop 66b and 660 and issupplied to relay driver 72, which drives dead time relay 62.

FIG. 7(g) shows the current waveform supplied to coil 56 by currentsource 60.

FIG. 3(b) shows in detail another circuit which may be used for controllogic circuit 54 in order to produce a measuring cycle extending over 64heart beats wherein during each half cycle four beats are used to allowstabilization of current during reversal and 28 beats are used tomeasure the blood flow signal during current steady-state conditions.Thus, during each measuring cycle 28 negative and 28 positiveelectrocardiogram components are cancelled out in averager S2 and 56positive blood flow pulses are loaded on the condensers of averager 52.

As shown, the control logic circuit comprises a chain of flip-flops 78a,78b, 78c, 78d, 78e, 78f sequentially connected together. The voltagesignal produced at the output of flip-flop 78f if used to trigger relay62 and generator 58 by means respectively of drivers 80 and 82. Thevoltage signal to trigger dead time relay 62 is derived from flip-flops78c, 78d, and 78e applied to relay 82 through NAND gate 84 and driver88.

FIG. 4 shows schematically one circuit that may be used for trapezoidfunction or ramp generator 58. A source of DC such as battery 90 isconnected to DPDT switch 92 which is connected to operational amplifier94 by means of adjustable resistor 96 which is connected to the negativeinput terminal of amplifier 94 and through ground to the positive inputterminal thereof. A capacitor 98 is connected between the nega tiveinput and output terminals of amplifier 94. Clipping circuit 100 isconnected to the output of amplifier 94.

In operation, when a trigger signal is received from logic circuit 54 toreverse DPDT switch 92, the voltage applied to amplifier 94 from battery90 is also reversed.

If it is assumed that the gain of amplifier 94 is high, its inputimpedance is high and its output impedance is low, then the shape of theoutput voltage will be determined by the values of resistor 96 andcapacitor 98 according to the following formula,

where R is the value of resistor 96 C is the value of capacitor 98 E isthe value of the voltage of battery E will have a slope determined bythe value of l/RC. Thus, by changing the value of variable resistor 96,the slope of E may also be changed.

Clipping circuit determines the voltage at which the ramp is stopped andrepresents the steady state voltage which is supplied to current source60.

FIG. 5 shows schematically a circuit that may be used for current source60. The function of current source 60 is to supply a constant DC currentto coil 56 in order to produce a constant magnetic field during theperiod when blood flow is being measured. It is also desirable that thecurrent through coil 56 be stabilized before the measurement intervalbegins.

Current source 60 basically comprises operational amplifier 102 incascade with power amplifier 104 the output of which is connected tocoil 56. Suitable input resistors 106, 108 and 110 and feedbackresistors 112 and 114 well known to those skilled in the art are alsoprovided and will not be described in detail here. As shown, the outputof ramp generator 58 is connected to the positive input terminal ofamplifier 102 by means of resistor 106.

In order to speed up the time in which the current through coil 56reaches a steady state value resistor 116 is provided in the outputcircuit of amplifier 104. Since the current passing through coil 56 alsopasses through resistor 116, a negative voltage is developed acrossresistor 116 which is fed back to the input to effectively decrease thenatural time constant of coil 56. Thus the current on reversal reachesits steady state value quicker than if resistor 116 were not provided.FIG. 6 illustrates the waveforms of the driving voltage and current onreversal.

FIG. 11 shows an electrode which may be used as the sensor electrode 26according to the present invention. The purpose of the electrode is toobtain the maximum electrical contact with the skin on which it isplaced. Electrode 118 comprises cup shaped plastic housing 120 havingmetallic electrode film 122 on the interior thereof covered withconductive jelly 124. Electrode film 122 is'of a non-polarizable naturesuch as a silversilver chloride lattice. Lead wire 126 is soldered toelectrode film 122 and carried out through housing 120. The electrodemay be secured to the skin of a patient by a suitable adhesive 128 whichmay be coated on the bottom of housing 120.

The conductive jelly 124 will mold itself to imperfections in the skinand provide maximum contact between the skin and electrode film 122.

FIG. 12 diagrammatically illustrates the optimum placement of electrodes26 with respect to the artery under measurement. Ideally, the magneticfield B is perpendicular to the skin and homogeneous in the region ofthe artery. The ideal position 1? at which one of the electrodes shouldbe placed is that point above the artery where the induced electricalvoltage is at a maximum or minimum. This point may be derived asfollows:

The voltage V, at point P on the skin surface is given by the followingformula B 1 2X =r e m where:

B is the magnetic field strength in Webers per square meter V, is thevoltage at point P in volts Q is the blood flow in the artery in cubicmeters per second 01 is the electrical conductivity of blood 02 is theelectrical conductivity of tissues V, is the slip velocity at the arterywall in meters per second, which is known to be very close to zerobecause of the laminar flow conditions A is the radius of the artery inmeters S is the distance from the center line of the artery to the skinsurface in meters X is the distance on the skin from point P to a linepassing through the center of the artery in meters The formula showsthat maximum and minimum voltages occur at two points on either side ofthe artery where X S. Thus, optimally, electrodes 26 should be placed atthese points in order to achieve the greatest blood flow signal.

Although it is desirable that a patient remain in one position whileblood flow is being measured, this may not always be possible and thereis a likelihood especially in measurement of the blood flow in thecarotid arteries that the patient will move his head during measurementcycle. This movement will change the orientation of the blood vessel inthe magnetic field thus changing the strength of the signal picked up bythe electrodes.

FIG. 13 shows in simplified form a circuit which may be used to controlthe gain of the measurement system in response to moderate positionchanges of a patient. A pickup coil 130 is placed on the skin betweenelectrodes 26.

In order to assure that the blood flow signal provided to averager 132is maintained at a constant level despite changes of position of theartery and skin in the magnetic field, a voltage divider networkcomprising photoconductive cell 134 and resistor 136 is connected to theinput of the averager. Cell 134 is enclosed within casing 138 with abulb 140. Coil 130 is connected to the filament of bulb 140 by means ofamplifier 142 and integrator 144.

A blood flow signal path is provided from electrodes 26 to averager 132through amplifier 146 and the variable resistance of cell 134.

In operation, when there is a change in position by the patient, coil130 will be moved in the magnetic field and an emf dv/dt will be inducedin coil 130. This emf is amplified by amplifier 142 and integrated inintegrator 144 to produce a voltage proportional to position. Thisvoltage is applied to the filament of bulb 140 and controls thebrightness thereof which in turn determines the resistance ofphotoconductive cell in the voltage dividing network. In this manner,when the flow induced signal picked up by electrodes 26 is weak, thegain of the circuit will be increased, and when the signal is strong,the gain will be decreased thus maintaining a signal of constantstrength to averager 132.

Since generally it is only necessary to monitor the blood flow in anygiven artery every 20 to 30 minutes, and since the measurement intervalby the flowmeter described hereinabove may only be 5 or 10 minutes andcan be reduced if the magnetic field is increased, it is possible to usethe same averager and indicator system for sequentially monitoring theblood flow in several vessels. In such a case, a pair of electrodes mustbe provided for each artery to be studied, and means must be provided toswitch the input of amplifier 44 from one pair of electrodes to the nextone sequentially according to a cycle program.

The above-described electromagnet 18 for creating a homogeneous magneticfield in the region of the artery under measurement is comprised of anelectrical coil wherein the current is maintained stable during allperiods of measurement. Other devices for producing a stable andhomogeneous magnetic field can be employed to function in the system ina manner similar to the electromagnet 18.

More specifically, referring to FIG. 14, there is shown an electricallyreversible permanent magnet 148 comprising a coil of wire 150 wound on apermanently magnetizable core 152 of magnetic material. The magneticcore 152 may be made of any alloy capable of retaining a sufficientpermanent magnetization, and for which the magnetization is not toodifficult to reverse. One such suitable alloy is Alnico No. 5.

A pulse of DC current produced by the measurement and control system154, is applied to the coil 150. via line 156, to saturate the core 152with a magnetization of a given polarity. The current need not be oflong duration, a second or two being sufficient, but it is preferable toraise the value of that current slowly enough to avoid the production ofunwanted surges of eddy currents in the human body. After themagnetization current is removed, the core 152 retains a stablemagnetization and the desired number of blood flow cycles can bemeasured. When this has been accomplished, the measuring circuit isparalyzed and a new pulse of DC current, of polarity opposite to thatpreviously used, is applied to coil 150 to dcmagnetize the core 152 andto remagnetize it with opposite polarity, after which the measuringcircuit is reversed and reactivated and a new series of blood flowcycles is measured. In this fashion, the reversible permanent magnet 152provides a stable and sufficiently homogeneous magnet field in theregion of the artery under measurement. This magnet field has itspolarity periodically reversed after a predetermined number of bloodflow cycles have been measured. v

The electromagnet 148 includes a pair of pole pieces in the form of softiron plates 158 and 160 supported at the respective ends of the magneticcore 152. The plates 158 and 160 cause a magnetic field distributionvery similar to that produced by coils having about the same diameter asthe end plates.

The energizing current can be obtained from a storage battery, by meansof a suitable relay system. Alternately, the polarity of magnetizationcan be reversed by feeding the coil 150 from a battery of condensers(not shown) which are charged during the time the current is not appliedto the coil 150, and discharged in the coil for short durations when themagnetic field is to be reversed.

One advantage afforded by the reversible permanent magnet 148 is thereduction in coil power consumption and, consequently, reduced heatdissipation resulting from the application of energizing current duringonly a small part of the measuring cycle, for example, four seconds outof one minute. Also, a stable magnetic field is produced by theelectromagnet 148 after the energizing current is removed. Therefore,stabilization circuits are not required, as would otherwise be employedwhen using an electromagnet having a soft iron core with an energizingcurrent applied during the entire measuring cycle.

Referring to FIG. 15, there is shown another type of permanent magnetdevice 162 which can be used to produce a stable and homogeneousmagnetic field in the area of the artery under measurement, Here, apermanent magnet 164 is employed which is mechanically displaced after afixed number of blood flow pulses have been measured and averaged, toprovide a magnetic field of opposite polarity in the region of theartery, such magnetic field being applied while an equal number ofpulses are measured and added in the averager. The permanent magnet 164is rotatably mounted on a shaft 166 which is supported on bearings 168and 170. Shaft 166 is driven on demand by motor 172 through a reductiongear system 174. Motor 172 is activated by control circuits within themeasurement and control system 176. Magnet 164 and its drive parts aremounted on a rigid frame 178 which is supported in a fixed position withrespect to the artery in which the blood flow is to be measured.

An auxiliary pole piece, such as the soft iron plate 180 shown in FIG.15, provides a desirable magnetic field distribution in the area of theartery under measurement. Plate 180 is rigidly positioned with respectto the axis of rotation of the magnet 164, while leaving a small air gap181 between the plate 180 and the magnet 164. Plate 180 is supportedadjacent to the human body where the artery under investigation islocated, thereby stably positioning the magnet 164 with respect to thebody and maintaining a substantially constant magnetic field in the areaof the artery by minimizing the movement of the body with respect to themagnet device 162.

After a number of cycles has been recorded with the magnet 164 in theposition indicated in FIG. 15, with the north pole 182 up, and the southpole 184 down, the magnet is rotated 180 degrees by the shaft 166 byaction of the motor 172 and the system 176. As with the use of theelectromagnet 148, shown in FIG. 14, it is desirable not to change themagnetic field too fast in order to avoid generating large eddy currentsin the body. Therefore, the half turn revolution of the magnet 164 takesbetween a few seconds, or as much as ten seconds, depending upon theconditions.

After the magnet 162 has been reversed while the measuring circuit isparalyzed, an equal number of cycles are measured with the new polarityto cancel the accumulated effect of the local cardiogram while adding tothe blood flow signal in the averager 52.

Although the above description is directed to a preferred embodiment ofthe invention, it is noted that other variations and modifications willbe apparent to those skilled in the art and, therefore, may be madewithout departing from the spirit and scope of the pres ent disclosure.For example, in measurement situations where the artery underinvestigation is located close to a relatively larger artery having asubstantially larger flow, the voltage generated by the blood flow inthe larger artery may contribute an undesirable component to the signalappearing between the measuring electrodes. Such undesirable blood flowcomponent is not eliminated by the averaging process because it issynchronous with the desired blood flow signals. This situation can beimproved if the two measuring electrodes, not shown, are implanted closeto the artery, and on opposite sides of it, instead of being placed onthe skin surface and spaced further apart, as described above inconnection with FIGS. 11 and 12. Since the blood flow generated voltageacross the artery is generally five or even ten times larger than thevoltage that can be picked up by skin electrodes, the sensing devicebecomes proportionally more sensitive to the blood flow. Also, theimplanted electrodes, which can be located very close to the arteryunder measurement, are set apart at a smaller distance from each otherthan would be the case for properly placed skin electrodes, the sensingdevice becomes less sensitive to the unwanted signals. The implantedelectrodes can be of the injectable type, such as th subcutaneouselectrodes known as subtrodes marketed by Siemens of West Germany, whichrequire no surgery but rather are implanted sim' ply by means of a formof hypodermic syringe.

Another technique for decreasing the contribution of the undesirableblood flow signals from the larger artery located near the artery underinvestigation is to use a combination of electromagnets, or reversiblepermanent magnets, adapted to create at the larger artery a magneticfield substantially lower than that created at the artery underinvestigation.

It is to be understood that, while the electromagnetic flowmeterdescribed above in reference to the present invention is particularlyadvantageous for use in noninvasive blood flow measurements, suchelectromagnetic flowmeter can also be adapted for use in those caseswhere it is necessary to implant the electrodes by surgery, for example,when measuring blood flow in deep vessels. In such cases, the advantagesof the described method are still considerable, one of them being thatthe magnetic field is generated by outside means and, consequently, noheat is generated in the tissues around the vessel.

Also, although the electromagnetic flowmeter according to the presentinvention has been particularly described in connection with an analogaverager, an analog-to-digital converter in combination with a digitalaverager or a digital computer properly synchronized by the signal ofthe reference electrocardiogram may be substituted for the analogaverager.

Furthermore, it is noted that while the electromagnetic flowmeteraccording to the present invention has been particularly described asproducing a stable and homogeneous magnetic field which is reversed inpolarity after a given number of heart cycles, an alternate approach isto suppress, instead of reverse, the magnetic field when the polarity ofthe electrical measuring circuit is reversed. When employing themagnetic field suppression technique, a larger magnet providing, forexample, twice the magnetic field intensity can be used producingsubstantially the same signal to noise ratio as in the case where themagnetic field polarity is reversed. Instead of doubling the magneticfield intensity, the averager can record the blood flow pulses for agreater number of pulses, for example, four times as many, before themagnetic field is suppressed. In this fashion, the operation ofsuppressing the magnetic field and quadrupling the recording timeprovides substantially the same signal to noise ratio as would otherwisebe present in the case where the magnetic field polarity is reversed,rather than being suppressed. Generally, the magnetic field reversaltechnique is more desirable than the above described methods wherein themagnetic field is suppressed, because the latter methods require eithera source of greater magnetic field intensity or, alternately, a longerperiod of averaging.

What is claimed is:

1. An electromagnetic flowmeter for measuring the blood flow pulse in ablood vessel ofa living being comprising: first sensing means adapted tobe positioned on the skin of the being at a location where a strong andsharp cardiogram pulse can be repeatedly obtained to be used as asynchronizing signal and as a clock; means connected to said firstsensing means for providing a synchronizing signal and a clock; apermanent magnet for producing a strong and homogeneous magnetic fieldin the region of said vessel, means for reversing said magnetic fieldafter a given number of heart cycles in accordance with a programcontrolled by said synchronizing signal, said magnetic field reversingmeans including a magnetizing coil through which a short duration pulseof DC. current is applied to reverse the polarity of said permanentmagnet; second sensing means adapted to be placed on the skin or in thetissue of said being at a location adjacent said vessel; amplifier meansfor amplifying the signal sensed by said second sensing means; means forreversing the polarity of said amplifier means during said reversal ofsaid magnetic field; and measuring means synchronized by thesynchronizing signal for averaging the pulsatile signal sensed by saidsecond sensing means during a given number of heart cycles while themagnetic field is stable at one polarity, and for averaging an equalnumber of heart cycles after reversal of said polarity of both saidmagnetic field and said measuring circuit; whereby there is obtained thewave shape of the blood flow pulses free from both the local cardiogramand the random noise.

2. The electromagnetic flowmeter of claim 1, including a source of DCcurrent pulses for reversing the magnetization of said coil.

3. The electromagnetic flowmeter of claim 2, also including a pair ofpole pieces supported at the respective ends of said permanent magnet.

4. An electromagnetic flowmeter for measuring the blood flow pulse in ablood vessel of a living being comprising: first sensing means adaptedto be positioned on the skin of the being at a location where a strongand sharp cardiogam pulse can be repeatedly obtained to be used as asynchronizing signal and as a clock; means connected to said firstsensing means for providing a synchronizing signal and a clock; apermanent magnet for producing a strong and homogeneous magnetic fieldin the region of said vessel; means for reversing said magnetic fieldafter a given number of heart cycles in accordance with a programcontrolled by said synchronizing signal, said magnetic field reversingmeans including means for physically moving said permanent magnet from afirst position to a second position whereby the polarity of saidmagnetic field is reversed; second sensing means adapted to be placed onthe skin or in the tissue of said being at a location adjacent saidvessel, amplifier means for amplifying the signal sensed by said secondsensing means, means for reversing the polarity of said amplifier meansduring said reversal of said magnetic field, and measuring meanssynchronized by the synchronizing signal for averaging the pulsatilesignal sensed by said second sensing means during a given number ofheart cycles while the magnetic field is stable at one polarity, and foraveraging an equal number of heart cycles after reversal of saidpolarity of both said magnetic field and said measuring circuit; wherebythere is obtained the wave shape of the blood flow pulses free from boththe local cardiogram and the random noise.

5. The electromagnetic flowmeter of claim 4, wherein said permanentmagnet is driven by a motor controlled in accordance with saidsynchronizing signal.

6. The electromagnetic flowmeter of claim 4, including a pole piece madeof highly permeable material and supported in a fixed position close tothe vessel under investigation, said pole piece supported adjacent saidpermanent magnet in a position which provides a small air gap betweenthe end of said pole piece and said permanent magnet during the periodsof measurement.

7. The electromagnetic flowmeter of claim 6.,

wherein said pole piece is made of soft iron.

1. An electromagnetic flowmeter for measuring the blood flow pulse in ablood Vessel of a living being comprising: first sensing means adaptedto be positioned on the skin of the being at a location where a strongand sharp cardiogram pulse can be repeatedly obtained to be used as asynchronizing signal and as a clock; means connected to said firstsensing means for providing a synchronizing signal and a clock; apermanent magnet for producing a strong and homogeneous magnetic fieldin the region of said vessel, means for reversing said magnetic fieldafter a given number of heart cycles in accordance with a programcontrolled by said synchronizing signal, said magnetic field reversingmeans including a magnetizing coil through which a short duration pulseof D.C. current is applied to reverse the polarity of said permanentmagnet; second sensing means adapted to be placed on the skin or in thetissue of said being at a location adjacent said vessel; amplifier meansfor amplifying the signal sensed by said second sensing means; means forreversing the polarity of said amplifier means during said reversal ofsaid magnetic field; and measuring means synchronized by thesynchronizing signal for averaging the pulsatile signal sensed by saidsecond sensing means during a given number of heart cycles while themagnetic field is stable at one polarity, and for averaging an equalnumber of heart cycles after reversal of said polarity of both saidmagnetic field and said measuring circuit; whereby there is obtained thewave shape of the blood flow pulses free from both the local cardiogramand the random noise.
 2. The electromagnetic flowmeter of claim 1,including a source of D.C. current pulses for reversing themagnetization of said coil.
 3. The electromagnetic flowmeter of claim 2,also including a pair of pole pieces supported at the respective ends ofsaid permanent magnet.
 4. An electromagnetic flowmeter for measuring theblood flow pulse in a blood vessel of a living being comprising: firstsensing means adapted to be positioned on the skin of the being at alocation where a strong and sharp cardiogam pulse can be repeatedlyobtained to be used as a synchronizing signal and as a clock; meansconnected to said first sensing means for providing a synchronizingsignal and a clock; a permanent magnet for producing a strong andhomogeneous magnetic field in the region of said vessel; means forreversing said magnetic field after a given number of heart cycles inaccordance with a program controlled by said synchronizing signal, saidmagnetic field reversing means including means for physically movingsaid permanent magnet from a first position to a second position wherebythe polarity of said magnetic field is reversed; second sensing meansadapted to be placed on the skin or in the tissue of said being at alocation adjacent said vessel, amplifier means for amplifying the signalsensed by said second sensing means, means for reversing the polarity ofsaid amplifier means during said reversal of said magnetic field, andmeasuring means synchronized by the synchronizing signal for averagingthe pulsatile signal sensed by said second sensing means during a givennumber of heart cycles while the magnetic field is stable at onepolarity, and for averaging an equal number of heart cycles afterreversal of said polarity of both said magnetic field and said measuringcircuit; whereby there is obtained the wave shape of the blood flowpulses free from both the local cardiogram and the random noise.
 5. Theelectromagnetic flowmeter of claim 4, wherein said permanent magnet isdriven by a motor controlled in accordance with said synchronizingsignal.
 6. The electromagnetic flowmeter of claim 4, including a polepiece made of highly permeable material and supported in a fixedposition close to the vessel under investigation, said pole piecesupported adjacent said permanent magnet in a position which provides asmall air gap between the end of said pole piece and said permanentmagnet during the periods of measurement.
 7. The electromagneticflowmeter oF claim 6, wherein said pole piece is made of soft iron.